Conventionally, various kinds of system control are carried out in a radio communication system in order to perform communication with high quality and high efficiency. An example is a radio communication system in which the modulation method or error correction method is switched adaptively according to the state of the propagation path. In this kind of system, the current propagation path state is determined based on the SIR (Signal to Interference Ratio) of a signal that a transmitting/receiving apparatus receives from a communicating party, or the like.
In practice, when the propagation path state is good, information data undergoes digital modulation using an M-ary modulation method with a large information bearing capacity per symbol, such as 64 QAM, for example, and is transmitted with the addition of an error correction code with a high degree of redundancy as an error correction method. When the propagation path state is poor, on the other hand, information data undergoes digital modulation using a modulation method with a small information bearing capacity per symbol, such as 16 QAM or QPSK, for example, and is transmitted with the addition of an error correction code with a low degree of redundancy as an error correction method.
Thus, in this kind of radio communication system, it is necessary to communicate to the receiving side the modulation method used to modulate a transmit signal and the error correction method used for error correction processing. A receiving apparatus has a plurality of demodulation sections corresponding to a plurality of modulation methods, and a plurality of error correction decoding sections corresponding to a plurality of error correction methods, and restores the original data by executing demodulation processing and error correction processing in accordance with a communicated modulation method information signal and error correction information signal.
An example of a known frame configuration for arranging signals indicating a modulation method within a transmit frame is that described in “Development of an Adaptive Modulation System for Land Mobile Communications—Outline of the Scheme—”, Hamaguchi et al., IEICE Communications Society Conference B-5-135 pp. 388, September 1997.
FIG. 1 shows this frame configuration. Located in order from the start of the frame are one ramp symbol R, six synchronization symbols SW, one pilot symbol P, one modulation method information symbol I, and 18 data symbols DATA.
A receiving apparatus that receives a signal with this kind of frame configuration determines the modulation method of data symbols DATA from modulation method information symbol I, and demodulates data symbols DATA using a demodulation method corresponding to the determined modulation method.
If, in the receiving apparatus, determination of modulation method information symbol I or an error correction method information symbol (not shown) is incorrect, the data obtained by demodulation and decoding of data symbols DATA will be incorrect. It is therefore important for modulation method information and error correction method information to be transmitted to the receiving apparatus correctly.
However, in radio communications, there is a risk of modulation method information and error correction method information being erroneously determined on the receiving side due to noise, or fading caused by propagation path fluctuations, resulting in a reduction in communication quality.